The invention relates to a balance based on the principle of the electromagnetic compensation of force with a cylindrical permanent magnet system, with a magnet cover which consists at least partially of a material with high permeability and has the function of magnetically screening the permanent magnet system, with a coil in the air gap of the permanent magnet system for generating the load-dependent counteracting force, with an optical position sensor and with an automatic gain control amplifier for regulating the current through the coil.
Balances of this type are generally known and are described, for example, in DE-GM No. 84 16 544. Emitter and receiver of the optical position sensor are located in this publication on the system carrier, which is fixed to the housing, whereas the slit diaphragm of the optical position sensor is fastened at the end of the translation lever. The principle of the optical position sensor is shown in FIG. 1 by way of explanation: An emitter 102 emits light which passes through slit 107 in slit diaphragm 106 partially onto the two light-sensitive surfaces 104, 105 of optical receiver 103. Emitter 102 and receiver 103 are fastened to parts 101 fixed to the housing, whereas slit diaphragm 106 is connected to the movable parts of the balance and moves with them, in a vertical direction in FIG. 1; this causes the distribution of light onto the two light-sensitive surfaces 104, 105 to change and an electric differential signal results which is proportional to the deflection of slit diaphragm 106. (The form and design of the slit are not essential for the function; even a round perforation in a sheet or a light-permeable area in a coated glass plate which is otherwise non-permeable to light, to cite only two examples, are to be understood under the term "slit diaphragm". Likewise, the term "light" is intended to include infrared or ultraviolet light too, for example).
It is also known from DE-OS No. 30 33 272 that emitter and receiver of the optical position sensor can be fastened on the permanent magnet system.
A disadvantage of all these known designs is the fact that the optical position sensor, which should determine the position of the coil in the air gap of the permanent magnet system, is relatively far removed from the latter. This can result e.g. due to temperature changes and/or temperature gradients in a relatively shifting between coil and position sensor, which adversely influences the precision of the balance.